The present invention generally relates to a method of treating polymeric materials, such as biomedical devices and contact lenses. In particular, the present invention is directed to a method of forming a coating onto a mold for forming a device and thereafter transferring the coating to the device as the device is formed within the mold.
Many devices used in biomedical applications require that the bulk of the device have one property, while the surface of the device has another property. For example, contact lenses may have high oxygen permeability through the lens to maintain good corneal health. However, materials that exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are typically hydrophobic and will adhere to the eye. Thus, a contact lens generally has a core or bulk material that is highly oxygen permeable and hydrophobic, and a surface that has been treated or coated to increase hydrophilic properties, thereby allowing the lens to freely move on the eye without adhering excessive amounts of tear lipid and protein.
In order to modify the hydrophilic nature of a relatively hydrophobic contact lens material, a contact lens can be treated with a plasma treatment. For example, a high quality plasma treatment technique is disclosed in PCT Publication No. WO 96/31793 to Nicolson et al. Some plasma treatment processes, however, require a significant monetary investment in certain equipment. Moreover, plasma treatment requires that the lens be dry before exposure to the plasma.
Thus, lenses that are wet from prior hydration or extraction processes must be dried, thereby imposing added costs of obtaining drying equipment, as well as added time in the overall lens production process. As a result, a number of methods of altering the surface properties of polymeric biomaterials, such as contact lenses, have been developed. Some of these techniques include Langmuir-Blodgett deposition, controlled spin casting, chemisorption, and vapor deposition. Useful examples of Langmuir-Blodgett layer systems are disclosed in U.S. Pat. Nos. 4,941,997; 4,973,429; and 5,068,318.
A more recent technique used for coating electronic devices is a layer-by-layer (xe2x80x9cLbLxe2x80x9d) polymer absorption process, which is described in xe2x80x9cInvestigation of New Self-Assembled Multilayer Thin Films Based on Alternately Adsorbed Layers of Polyelectrolytes and Functional Dye Moleculesxe2x80x9d by Dongsik Yoo, et al. (1996). The process described in this article involves alternatively dipping hydrophilic glass substrates in a polyelectrolyte solution (e.g., polycations such as polyallylamine or polyethyleneimine) and then in an oppositely charged dye solution to form electrically conducting thin films and light-emitting diodides (LEDs). After each dipping, the substrates are rinsed with acidic aqueous solutions. Both the dipping and rinsing solutions have a pH of 2.5 to 7. Prior to dipping, the surfaces of the glass substrates are treated in order to create a surface having an affinity for the polyelectrolyte.
Similar to the above process, two other processes are described by 1995 publications entitled xe2x80x9cMolecular-Level Processing of Conjugated Polymersxe2x80x9d by Fou and Rubner and Ferreira and Rubner, respectively. These processes involve treating glass substrates that have hydrophilic, hydrophobic, negatively, or positively charged surfaces. The glass surfaces are treated for extended periods in hot acid baths and peroxide/ammonia baths to produce a hydrophilic surface. Hydrophobic surfaces are produced by gas-phase treatment in the presence of 1,1,1,3,3,3-hexamethyldisilazane for 36 hours. Charged surfaces are prepared by covalently anchoring charges onto the surface of the hydrophilic slides. For example, positively charged surfaces are made by further treating the hydrophilic surfaces in methanol, methanol/toluene, and pure toluene rinses, followed by immersion in (N-2 aminoethyl-3-aminopropyl) trimethyloxysilane solution for 12 to 15 hours. This procedure produces glass slides with amine functionalities, which are positively charged at a low pH.
In addition to the above-described techniques, U.S. Pat. Nos. 5,518,767 and 5,536,573 to Rubner et al. describe methods of producing bilayers of p-type doped electrically conductive polycationic polymers and polyanions or water-soluble, non-ionic polymers on glass substrates. These patents describe extensive chemical pre-treatments of glass substrates that are similar to those described in the aforementioned articles.
The methods described above generally relate to layer-by-layer polyelectrolyte deposition. However, these methods require a complex and time-consuming pretreatment of the substrate to produce a surface having a highly charged, hydrophilic, or hydrophobic nature in order to bind the polycationic or polyanionic material to the glass substrate.
To reduce the complexity, costs, and time expended in the above-described processes, a layer-by-layer polyelectrolyte deposition technique was developed that could be effectively utilized to alter the surfaces of various materials, such as contact lenses. This technique is described in U.S. patent application Ser. No. 09/199,609 filed on Nov. 25, 1998. In particular, a layer-by-layer technique is described that involves consecutively dipping a substrate into oppositely charged polyionic materials until a coating of a desired thickness is formed. Nevertheless, although this technique provides an effective polyelectrolyte deposition technique for biomaterials, such as contact lenses, a need for further improvement still remains. For example, one way to manufacture contact lenses is to dispense a substrate into a mold, and thereafter cure the mold such that the substrate becomes polymerized and forms a contact lens. After polymerizing the substrate, it can then be removed and coated as described above. However, the substrate can often become adhered to the mold such that it is destructively torn upon removal. Moreover, it is often difficult to handle the delicate lens after it is formed such that it can be coated as described above.
As such, a need currently exists for an improved method of coating a material, such as a contact lens, with polyelectrolyte (polyionic) layers. In particular, a need exists for an improved method of forming a polyionic coated contact lens in a mold without destructively tearing the lens from the mold upon removal and for allowing the lens to be treated upon removal from the mold.
Accordingly, an object of the present invention is to provide an improved method of treating a contact lens to alter surface properties.
It is another object of the present invention to provide a method of coating a mold with polyionic materials, such as polyanionic and polycationic materials.
Still another object of the present invention to provide an improved method of forming a contact lens within a mold.
Another object of the present invention is to provide a method of coating a mold with polyionic materials and thereafter forming a contact lens within the mold such the lens becomes coated with the polyionic materials.
These and other objects of the present invention are achieved by providing a method for applying polyionic materials to a mold used in forming polymeric substrates, such as contact lenses. The method of the present invention can apply successive layers of polyionic material onto a mold using various techniques, such as spraying, multi-step dipping, or dipping in a single solution.
In accordance with the present invention, a coating can be applied to a mold used in forming contact lenses. In general, a mold can be formed by any method known in the art, such as by injection molding. Typically, two mold halves are formed and later joined together such that a cavity can form therebetween. Although it is typically desired that the mold be made from a material having at least some affinity to polyionic materials, virtually all materials known in the art for making molds can be used. For example, various types of thermoplastic material, such as UV-transmissive or UV-opaque thermoplastic materials, can be utilized to form a mold of the present invention. In one embodiment, one portion of the mold is formed from a UV transmissive material, such as polymethylacrylate, so that UV light can later pass through the section to cure a polymerizable material dispensed within the mold. In another embodiment, another portion of the mold is formed from a UV-opaque material that blocks UV light.
As stated, once a mold is formed, negatively and positively charged materials, such as polyionic materials can then be applied to the mold to form a coating thereon. In general, at least one polyanionic material and at least one polycationic is utilized, although more than one of each polyionic material can be employed.
Typically, a polycationic material of the present invention can include any material known in the art to have a plurality of positively charged groups along a polymer chain. For example, in one embodiment, the polycationic material includes poly(allyl amine hydrochloride). Likewise, a polyanionic material of the present invention can typically include any material known in the art to have a plurality of negatively charged groups along a polymer chain. For example, in one embodiment, the polyanionic material includes polyacrylic acid.
In addition to polyionic materials, various other materials and/or additives can be applied to a mold of the present invention before forming a contact lens therein. Some examples of suitable additives include, but are not limited to, antimicrobials, antibacterials, visibility tinting agents, iris color modifying dyes, ultraviolet light tinting dyes, cell growth inhibitors, etc.
According to the present invention, polyionic materials and/or other additives can be applied to a mold using any technique known in the art for applying a coating to a material. For example, the polyionic materials can be spray coated onto the mold using one or a series of spray coating techniques. One such spray coating technique that can be used in the present invention is described in U.S. Pat. No. 5,582,348 to Erickson et al., which is incorporated herein by reference. This particular spray technique involves the atomization of a liquid by ultrasonic vibrations such that a spray can form therefrom. It should be understood, however, that any other spray coating technique known in the art can also be used in the present invention.
In addition to spray coating, dipping techniques can also be used to apply polyionic materials and/or other additives to a mold. One dipping technique that can generally be used in the present invention is multi-step dipping process, such as described in U.S. patent application Ser. No. 09/199,609, which is incorporated herein in its entirety by reference thereto. In particular, multi-step dipping processes involve the consecutive application of oppositely charged polyionic materials onto a material. Besides multi-step dipping, a single-dip process, such as described in U.S. patent application (filed on the same day as the present application) entitled xe2x80x9cSingle-Dip Process for Achieving a LbL-Like Coatingxe2x80x9d can also be used in the present invention to apply a polyionic coating onto a mold. In some embodiments, combinations of the above mentioned coating techniques, as well as other well known coating techniques, can be used.
Moreover, in one embodiment, the mold can be xe2x80x9cpreconditionedxe2x80x9d to enhance the ability of the polyionic solution to coat the mold. For example, a standard layer-by-layer process can be used to form an underlayer or primer coating on the substrate. This underlayer can sufficiently xe2x80x9croughenxe2x80x9d the surface such that the ultimate polyionic solution of the present invention can better adhere to the mold surface.
Once a mold is sufficiently coated in accordance with the present invention, a substrate material can then be dispensed into a cavity formed by the connection of the mold halves. In general, a substrate material of the present invention can be made from any polymerizable material. In particular, a substrate material of the present invention can be made from oxygen-permeable materials. For example, some examples of suitable substrate materials include, but are not limited to, the polymeric materials disclosed in U.S. Pat. No. 5,760,100 to Nicolson et al., which is incorporated herein by reference.
After the substrate material has been dispensed within the mold, it can then be cured using processes well known in the art. Curing the substrate material within the mold causes the material to become polymerized. Morever, the polyionic materials applied to the mold become detached during curing and reattach to the polymerized contact lens. As such, by coating a mold according to the present invention, the coating can be transferred onto the contact lens formed therein. The coated contact lens can thereafter be easily removed from the mold without destructively tearing. Moreover, further substantial handling of the contact lens is not required.
In some embodiments, however, it may be desired to apply secondary coatings of polyionic materials and/or other additives to the contact lens after it is formed. As such, any of the coating techniques discussed above can be utilized to further coat the contact lens. Moreover, in some embodiments, the formed contact lens can be xe2x80x9cpreconditionedxe2x80x9d before applying such secondary coatings.
As described above, one method for xe2x80x9cpreconditioningxe2x80x9d a contact lens is by the application of a primer layer or underlayer of coating. In addition, a solvent solution comprising a solvent and at least one polyionic material can also be applied to the contact lens for preconditioning. The application of a solvent, such as an alcohol solution, can allow the lens to swell. After swelling, the lens can then be removed from the solvent solution such that it shrinks. The shrinking of the lens can entrap the polyionic component(s) within the lens. As a result, in some embodiments, the ultimate polyionic solution can be more easily adhered to the lens surface when applied thereto.
Other objects, features and aspects of the present invention are discussed in greater detail below.